The problem with the grommets is not necessarily that they grip badly, but that it's super easy to accidentally just push them through when you hastily go to slap the side cover on. With the grommet loose or halfway loose you're in danger of losing a cover at any moment. So at the end of the day I applied tons of clear silicone to the edge of my grommets and just glued them in place with that, and also did the ziptie thing, at least then you have a solid chance of retaining the cover if it comes loose. The minimum size hole you can make in the rear post to thread a ziptie through shouldn't really affect its structural integrity much, I'm guessing. So far so good, anyway.
Also, you can buy a set of new grommets from World of Triumph if you want the exact thing (if they've dried and gone less tacky) - or for that matter go buy some extra beefy ones if you have access to a great hardware store or something (which I didn't, just couldn't find grommets that worked for it without buying the originals.)